Protective measures must be taken to ensure the safety of personnel operating, or those merely in the vicinity of, power presses and other like machines where it is impractical to impose mechanical barrier guards. The measures frequently used involve a requirement for providing means to sense the hands and other vulnerable body parts when the latter violate a minimum allowable space or safety margin with respect to the point of danger.
A familiar technique for sensing human or other electrically conductive parts is by capacitance. The principle of operation of this type of technique is based on the fact that the flesh of the operator is sufficiently electrically conducting to allow a properly connected sensitive bridge circuit to respond to increased capacitance, i.e., charge storage, afforded by the operator's close proximity to an antenna placed around the possible entry areas to the point of contact. However, previous capacitance sensing systems have either been too expensive, not sufficiently fail-safe, or too difficult to set up.
Insofar as concerns the matter of being fail-safe, many electronic devices, especially "binary" or digital, pulse and DC circuits use two information conditions, i.e., voltage states. It is convenient for one of the states to be at ground, "null", or near zero potential or energy condition and the other state to be near a set maximum potential. Unfortunately, in the currently popular binary systems component failures usually cause an indeterminate condition, i.e., a failure appears to be one normal state or another, and in some instances even though the circuit is no longer correctly functioning or able to change states a false impression is given. In AC circuits, many conditions also can appear like zero signal, often a normal state, when a component fails. It is, of course, possible to set up level detection circuits to define different signal levels and to be able to distinguish them from the OFF state. However, in a bridge-type circuit which is the best practical way to achieve the kind of sensitivity and stability required for this application, there will always be a zero level condition, which can not be distinguished from an "OFF" type failure condition. Hence, conventional null or zero level can not be used for the machine safe indication, if the sensing device is to be considered fail-safe.
Fail-safe is defined for the aforereferenced type of equipment in an American National Standards Document as "designed to revert to an OFF, or safeguarding condition, in the event of a power or component failure or other detrimental (condition)". There are three ways to design circuitry that can be said to achieve the fail-safe character. They are (1) to provide redundant circuits, (2) to provide monitoring circuits, or (3) to make the basic circuits intrinsically fail-safe. The intrinsic approach is by far the best because even though the first two provide for any single component's failure and many combinations of failure, it is theoretically possible for the redundant circuit and/or the monitoring circuit components to fail, as well as the basic circuit. To be truly fail-safe any component failure should result in a reaction from the instrument that disables the protected machine or operation.
Therefore, although it has been known heretofore to utilize capacitance sensing systems, the prior art forms thereof which have been provided heretodate have been characterized by the fact that they are either too expensive, not sufficiently fail-safe, or too difficult to set-up. A need has thus existed to provide a capacitance sensing system which is intrinsically fail-safe. Moreover, a desire has been expressed to provide such an intrinsically fail-safe safety system which minimizes the number of components needed to be embodied therein, particularly in comparison to redundancy and monitoring approaches which require extra components that increase the cost of providing and maintaining the system.
Accordingly, it is an object of the present invention to provide a novel and improved electronic personnel safety sensor capable of causing a machine or operation to be shut off to prevent injury to personnel or damage to equipment being protected.
It is also an object of the present invention to provide such an electronic personnel safety sensor which is operable to perform the desired protection function by detecting, indicating, and acting upon a predetermined increase or decrease in effective capacitance caused by the introduction, or intrusion, or withdrawal of material to or from a field or protection zone created by a suitably designed and constructed electrode structure.
It is another object of the present invention to provide such an electronic personnel safety sensor which embodies circuitry wherein all of the active units thereof are provided with an enabling signal which is derived by series passage through the system from a common source.
A further object of the present invention is to provide such an electronic personnel safety sensor which is intrinsically fail-safe for all components except the output relay for which redundancy and/or monitoring must be provided.
A still further object of the present invention is to provide such an electronic personnel safety sensor which requires for its implementation the inclusion in the circuitry of a minimum number of components.
Yet another object of the present invention is to provide such an electronic personnel safety sensor which is relatively inexpensive to manufacture, and relatively easy to set-up and deploy.